1. Technical Field
The present disclosure relates to an image display device, and more specifically to an image display device suitably applicable to, for example a configuration that projects an image formed by an image forming unit onto a screen with laser light from a light source, thereby displaying the image.
2. Description of the Related Art
In recent years, image display devices equipped with a virtual image optical system have been increasingly developed. The virtual image optical system displays image in space by using a reflective glass surface (or a half mirror), a free-form surface mirror, and other optical components in combination.
The above virtual image optical system is expected to be applied especially to image display devices, called head-up displays, to be mounted in passenger vehicles or other movable bodies. An exemplary head-up display to be mounted in a passenger vehicle modulates light based on image information and irradiates the windshield with the modulated light. Then, the modulated light is reflected by the windshield and incident on the driver's eyes. In this way, the driver can view a virtual image based on the image information in front of the windshield. For example, the virtual image displayed in this manner may contain a vehicle speed or an ambient temperature. Studies are currently being conducted to display, as virtual images, a navigation image and an image for calling driver's attention to passersby, for example.
Some head-up displays, as described above, are equipped with a laser light source including a semiconductor laser, as their light source. This configuration scans a screen with laser light modulated with an image signal. Then, the laser light is diffused over the screen so that the light is incident on the driver's eyes from a wider area. This prevents the driver's eyes from leaving the irradiated area even if he or she moves the head to some degree. Consequently, the driver can view the image (virtual image) appropriately and stably.
By using a laser as a light source, an optical system that scans the screen with a laser beam can be downsized. Furthermore, by combining a virtual image optical system having a high optical magnification with a laser light source, a smaller optical system can display a larger virtual image. These configurations are expected to enable head-up displays to be installed in many different types of vehicles.
However, when a laser, especially a semiconductor laser is used in an image display device as its light source, the output of the light source relative to its input power (electric-optic conversion efficiency) may decrease at certain temperatures. More specifically, if the image display device displays a white image, this white image might be darkened at certain temperatures. Likewise, if the image display device displays a single-colored image, such as a red, green, or blue image, this image might be darkened at certain temperatures.
Moreover, the image display device may fail to clearly exhibit gradations of colors at low luminance at certain temperatures, because a light-emitting property of a laser light source is fluctuated at or near its oscillation threshold.
Many engineers have considered measures against problems associated with an adjustment of an output of a laser light source as described above and have proposed some solutions. For example, Unexamined Japanese Patent Publication No. 2013-15738 discloses a method for facilitating gradation control. More specifically, when image is displayed at low luminance, a laser beam emitted from each laser light source passes through a polarization control element with its angle of polarization adjusted in accordance with a light quantity requirement. In this way, the light quantity from the laser light sources are adjusted so that the gradation is controlled.
Unexamined Japanese Patent Publication No. 2013-226746 discloses a method for rewriting data into a lookup table. In this method, light quantities of a laser light source are obtained at predetermined intervals in accordance with a temperature at the power-on. Then, an input-output property of the laser light source is linearly interpolated using the maximum and minimum of the light quantities obtained.
Unexamined Japanese Patent Publication No. 2014-186078 discloses a method for decreasing a temperature load on a laser light source. In this method, when its surrounding temperature rises, a device uses optical detectors to control currents flowing to respective laser light sources while maintaining white balance. More specifically, the device decreases a current flowing to a laser light source whose temperature has greatly risen so that the temperature falls. In short, the device decreases luminance while maintaining a white color.
Unexamined Japanese Patent Publication No. 2015-22096 discloses a method for finely controlling luminance and gradations. In this method, current flowing to a laser light source and a high-frequency signal wave to be superimposed on the current are adjusted.
Unexamined Japanese Patent Publication No. 2015-64539 discloses a method for controlling an output of a laser light source based on the combination of applied voltage and a light-emitting time.